Control devices for controlling an electrical direct current motor are known in which the motor is driven by applying an operating voltage supplied by the control device to the connection terminals of the motor (drive mode). By applying the operating voltage in alternating polarity the direction of rotation (right, left) of the motor can be determined or reversed. The speed of the motor can be varied in an open-loop control or closed-loop control system by a corresponding variation of the operating voltage or of the operating current applied. To bring the motor to a stop again it is often sufficient simply to cut the power to the motor since the motor is then braked under the force of friction.
However, if the motor is to be brought to a stop more quickly or “actively braked” this can be achieved by the connection terminals of the motor being short circuited (braking mode).
To initiate the braking mode or the short circuit, with known control devices a switch is closed (switched on), which short-circuits the connection terminals of the motor over a short-circuit path containing the switch.
An attempted or actual motor rotation caused by a load coupled mechanically to the motor can also be braked or restricted by such as short circuit.
The short circuit can also be removed after the motor comes to a stop and the polarity of the operating voltage at the connection terminals reversed, so that the motor starts up again in the opposite direction.
Such motors are typically used in automotive technology, in systems engineering and in household appliance technology. In practice the modes (drive mode, braking mode) or the directions of movement or rotation (to the right, to the left) generally change very often, viewed over the service life of the system.
Switches used for applying the operating voltage or for producing short circuits, e.g. pairs of relay switch contacts, are in many cases subjected to a significant (cumulative) switching load considered over the service life of the motor. During short circuiting in particular a heavy short-term load is imposed on the switches used for this purpose, if this switch is “hard switched”, i.e. an induction voltage is switched on under the load and then a comparatively large short circuit current flows over the switch. This is a problem to the extent that the switching contacts are subjected to significant wear by these loads.
This problem has already been resolved in the past for mechanical switches (e.g. relays) by using switching contacts which were designed to safely withstand the number of short-circuit switching processes expected over the service life of the motor. Such switches however are relatively expensive and need a relatively large amount of space. Another solution, but a relatively expensive one however, would be to use very powerful semiconductor switches.